1. Field of the Invention
This invention relates to metal oxides useful in making transparent electrodes and conductive layers, protective layers for such conductive layers and dielectric layers in flat panel displays, fluorescent lamps and other electrooptical devices, and more particularly to liquid precursors for making such transparent metal oxides and methods for making such precursors.
2. Statement of the Problem
A typical fluorescent lamp comprises a cylindrical glass tube or envelope containing mercury vapor and a phosphor layer covering the inside of the tube wall. Many fluorescent lamps, in particular rapid-start fluorescent lamps, usually comprise one or more transparent metal oxide layers; for example, an electrically conductive metal oxide layer on the inner surface of the glass tube, and a metal oxide protective layer between the conductive layer and the phosphor layer of the lamp.
A conventional technique of the art of forming transparent metal oxide layers in fluorescent lamps involves: dispersing a solid powder of the desired oxide in a liquid medium to make a colloidal suspension of the oxide; applying a coating of the suspension onto a surface of the lamp; and drying the coating to form the oxide layer. Generally, it is difficult to achieve a uniform, continuous thin film by applying a colloidal suspension of powdered particles. Another technique involves dissolving a precursor compound in a solvent and spraying the precursor solution onto a hot surface having a temperature above the crystallization temperature of the desired oxide, whereby the precursor compound is immediately pyrolyzed. A typical conventional precursor for a conductive layer contains tin tetrachloride, SnCl4, and hydrogen fluoride, HF, in butanol. The chlorine and fluorine are highly electronegative, salt-forming atoms that may lead to lamp defects called xe2x80x9cmeaslesxe2x80x9d, as described below. Further, highly reactive precursor compounds such as SnCl4 and HF are toxic and difficult to handle, and do not store well.
It is known in the flourescent lamp art to employ a protective layer of aluminum oxide, often called alumina, or certain other metal oxides, such as oxides of cerium, yttrium, titanium, and zirconium to inhibit or delay discoloration and other appearance defects in the phosphor layer or the conductive oxide layer. Silicon oxide, often referred to as silica, may also be included in a protective layer. These barrier layers of the prior art are located between the conductive oxide layer and the phosphor layer. The advantages of the protective coating are probably a result of the relatively nonporous metal oxide coating that protects the conductive oxide layer from ion bombardment resulting from arc discharge. The protective layer is generally formed utilizing an aqueous colloidal suspension or a dispersion of the metal oxide in a liquid. As indicated above, it is difficult to deposit a continuous solid layer using a colloidal suspension or dispersion. Another typical conventional precursor for a protective layer is a metal alkyl compound in a solvent, which is pyrolyzed. It is generally difficult to form a uniform, continuous metal oxide thin film by the conventional pyrolysis method of the prior art because pyrolysis of the sprayed precursor compound on the hot substrate results in a broken, uneven surface on the microscopic level.
Flat panel displays typically comprise one or more layers of transparent metal oxide thin films. Transparent conductive metal oxide thin films commonly serve as electrodes between the light source and the viewing surface to assist in establishing electric fields for operating the display. It is usual for thousands of metal oxide electrodes to be formed. The electrodes must necessarily be transparent so that they do not interfere with the intensity and quality of the light images produced for display. A flat panel display often also comprises one or more transparent nonconductive metal oxide thin films that serve as insulators or protective coatings in the display. These metal oxide thin films are typically deposited using sputtering techniques that are complex and unreliable. Conventional deposition techniques often result in problems, such as poor adherence of the thin films to their substrate and xe2x80x9cpinholexe2x80x9d defects in the deposited thin film.
The present invention provides novel precursors for forming transparent metal oxide thin film layers in a fluorescent lamp, a flat panel display or other electrooptical device. In particular, the invention provides novel nonaqueous metal organic liquid precursor solutions (xe2x80x9cliquid precursorsxe2x80x9d) and methods of making such precursors.
In one embodiment, the invention provides a novel liquid precursor for forming a transparent, electrically conductive metal oxide thin film layer (xe2x80x9cconductive layerxe2x80x9d), which may be used, for example, in a fluorescent lamp.
The invention further provides a novel liquid precursor for forming an electrically nonconductive metal oxide protective layer (xe2x80x9cprotective layerxe2x80x9d), which may be used, for example, between the phosphor layer and the conductive layer of a fluorescent lamp. The inventive metal organic precursors may be used to fabricate a wide variety of metal oxide thin film layers.
The invention further provides a novel liquid precursor for forming an electrically conductive metal oxide thin film as a transparent electrode in a flat panel display.
A nonaqueous metal organic liquid precursor solution (xe2x80x9cliquid precursorxe2x80x9d) of the invention is usually applied to a substrate surface using a liquid source deposition technique. The liquid precursor contains one or more metal organic precursor compounds that lead to formation of the desired metal oxide thin film layer upon reaction and crystallization on the substrate surface. Usually, the desired oxide is a metal oxide; therefore, the liquid precursor usually contains a metal organic precursor compound. The oxide formed by the inventive liquid precursor may also be a nonmetallic oxide, such as silicon oxide, in which case the organic precursor compound of the liquid precursor is also nonmetallic. An inventive liquid precursor is a solution of one or more organic compounds dissolved in a nonaqueous solvent. The precursor is applied to the substrate surface and treated, usually by one or more heating techniques. As a result, the organic precursor compound or compounds react to form a solid thin film having the desired composition on the substrate surface. In the fabrication of a fluorescent lamp, the lamp envelope may be dipped or rolled in liquid precursor to form a liquid coating of precursor, which is then treated; the liquid precursor solution may also be applied using a conventional liquid spraying method, as known in the fluorescent lamp art. In the fabrication of flat panel displays, fluorescent lamps and other electrooptical devices, the liquid precursor is often applied by a liquid misted deposition method, in which a very fine mist of liquid particles is formed in a carrier gas and deposited on the substrate surface.
A nonaqueous metal organic liquid precursor solution of the invention comprises an organic precursor compound containing a first metal selected from the group including tin, antimony, indium, niobium, tantalum, bismuth, cerium, yttrium, titanium, zirconium, hafnium, and silicon. In one embodiment, the first metal is selected from the group consisting of tin, antimony, indium to make a conductive metal oxide material that may include SnO2, Sb2O3, and In2O3. In this embodiment, the liquid precursor preferably further comprises an organic dopant precursor compound containing a metal selected from the group including niobium, tantalum, bismuth, cerium, yttrium, titanium, zirconium, hafnium, silicon, zinc and magnesium. In another embodiment of the invention, the liquid precursor comprises an organic precursor compound containing a metal selected from the group including cerium, yttrium, titanium, zirconium, hafnium, silicon, niobium, tantalum, and bismuth. Thus, the liquid precursor contains metal moieties in effective amounts for forming one or more oxides selected from the group including Ce2O3, Y2O3, TiO2, ZrO2, HfO2, SiO2, Nb2O5, Ta2O5, and Bi2O3.
According to the invention, the liquid precursor solution discussed above is applied to the substrate surface, and then a solid metal oxide is formed in heating steps subsequent to the liquid application step. In this regard, the invention has two significant aspects: 1) the liquid precursor is a solution of a metal precursor compound, not a colloidal suspension or dispersion of the material to be deposited; and 2) the liquid precursor solution has an opportunity to distribute itself on the substrate surface before a solid is formed. These aspects of the invention may be employed alone or in combination. Both of these aspects contribute to the formation of a continuous, uniform layer of solid on the substrate surface. Since the liquid is a solution of a metal compound, the distribution of the metal elements is inherently uniform in the liquid. Since the liquid has an opportunity to distribute itself before solidifying, advantage can be taken from the natural tendency of a liquid, as compared to a solid, to distribute itself evenly.
The metal organic liquid precursor solutions of the invention are chemically stable, so they can be prepared in advance and stored safely for relatively long periods, up to six months. Initial precursors for a plurality of different constituents and dopants may be dissolved in the same final liquid precursor, so that the composition of the metal oxide thin film layer can be easily varied and controlled.
The metal organic liquid precursors of the invention do not contain chlorine or other highly electronegative salt-forming atoms. Chlorine and other salt-forming atoms contained in precursors of the prior art are starting points of measle defects in fluorescent lamps.
The inventive nonaqueous metal organic liquid precursor solutions may be used to manufacture conductive layers using a wide variety of different metal organic precursor compounds to form a wide variety of metal oxide materials, with or without dopants. An inventive liquid precursor solution may include an organic dopant precursor compound.
The preferred liquid precursor used for making a conductive layer comprises a 0.5 molar solution of tin 2-ethylhexanoate and antimony 2-ethylhexanoate in 2-ethylhexanoic acid and n-octane, containing niobium 2-ethylhexanoate, in relative molar proportions corresponding to the stoichiometric formula Sn0.97Sb0.03O2 with 0.006 mole percent Nb2O5. In other conductive layers having good properties, the dopants may be Ta2O5 and Bi2O3. The composition of the inventive metal organic liquid precursor solutions and resulting metal oxide layers avoids the generation of measle defects during fluorescent lamp operation. The resistivity profile in a conductive layer fabricated using an inventive liquid precursor can be controlled by selection of liquid precursor composition and treating temperatures. It is understood that an inventive precursor may have a composition different from the preferred composition, and that an inventive precursor may be used to form a metal oxide thin film with a composition different from the preferred composition of thin film.
The novel precursors of the invention may be used advantageously to fabricate metal oxide protective layers in fluorescent lamps and other devices, usually comprising a single metal oxide, such as Y2O3 or Ce2O3. The inventive precursors may also be used to fabricate metal oxide thin film layers in fluorescent lamps besides a conductive layer or a protective layer. For example, the fabrication of some fluorescent lamps may include formation of an electrically nonconductive metal oxide layer on the lamp envelope surface between the envelope wall and a conductive layer.
Precursors according to the invention are also useful in the fabrication of flat panel displays. A flat panel display typically comprises one or more layers of transparent metal oxide thin films. Transparent thin films are important because they do not interfere with the display screen image when viewed from the front, or with the passage of backlighting from the back of the display. Transparent conductive metal oxide thin films commonly serve as electrodes between the light source and the viewing surface to assist in establishing electric fields for operating the display. In flat panel displays, transparent thin film layers of indium oxide, tin oxide, indium tin oxide (xe2x80x9cITOxe2x80x9d) or other conductive metal oxide may be formed using an inventive liquid precursor. A flat panel display often also comprises one or more transparent nonconductive metal oxide thin films that serve as dielectric insulators or protective coatings in the display. An inventive liquid precursor allows use of a liquid source deposition technique instead of conventional target sputtering or CVD techniques.
Metal organic liquid precursors according to the invention can be manufactured reliably. Their composition can be easily controlled and varied, if necessary. They can be safely stored for long periods, up to six months. They are relatively nontoxic and nonvolatile, compared to precursors of the prior art. Metal oxide thin film layers formed using liquid precursors of the invention have smooth, continuous and uniform surfaces, especially compared to oxide layers of the prior art. They can be reliably fabricated to have thicknesses in the range of 20-500 nm, maintaining important characteristics such as transparency and desired electrical properties.
The composition of a liquid precursor of the invention may be selected to provide a transparent metal oxide thin film having desired electrical properties, such as electrical conductivity, depending on a set of fabrication conditions. The electrical conductivity of a metal oxide thin film formed using an inventive precursor may vary with such factors as annealing temperature, annealing time, composition of ambient atmosphere during fabrication steps, and film thickness, among others. An important advantage of the invention is that the concentrations of a plurality of organic precursor compounds and organic precursor dopant compounds in an inventive liquid precursor may be easily and controllably varied and used in a deposition process to produce a desired metal oxide thin film for given fabrication steps.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.